Liquid scintillation counting is a generally known and widely used technique for the measurement of low energy beta emitting radionuclides in addition to gamma and alpha emitters. The liquid scintillation counter is utilized to make quantitative measurements of radioactivity when incorporating a radio-labeled analyte into solution with a liquid medium capable of producing photons resulting from the kinetic interactions of nuclear decay products. There are different radionuclides, both man-made and naturally occurring, which can be experimentally employed to illustrate the molecular dynamics of a particular model by measuring their distributions in various systems. It is often desirable to use multiple radionuclides in the same experiment to follow different distributions in the same system simultaneously.
The energy of the beta particles ranges from zero energy to the E.sub.max for that radionuclide. A pulse height energy distribution spectrum may be recorded for the solution being analyzed. When the solution being analyzed contains a mixture of beta emitters there is an overlap of the spectra from each emitter.
Tritium has developed immense importance in various applications wherein radio-labeled components are utilized. By dissolving a sample containing the tritium in a mixture with a typical liquid scintillation cocktail, such as an aromatic solvent containing an organic scintillator, energy from the beta decay is transferred with reasonable efficiency to the scintillators which emit multiple photons proportional to energy of the beta particle. The light is detected by sensitive photomultiplier tubes. In a normal state these photomultiplier tubes have a characteristic dark noise which occurs upon the spontaneous emission of an electron within the tube. The dark noise consists of pulses generating a low intensity photon response. Thus, by utilizing multiple photomultiplier tubes and by requiring a coincidental event, beta emitters can be measured while minimizing the background. The higher the energy of the beta particle, the more photons will be produced over selected time intervals such as 20-50 nanoseconds. All photons produced within that interval are considered a pulse. The combined amplitude of the photons is known as the pulse height. The range of pulse heights is a smooth distribution of energies ranging from zero energy rising to a maximum and tailing off to the maximum energy characteristic of the beta emitting radionuclide. This distribution is known as the pulse-height energy distribution spectrum.
FIG. 1 is a graph of a simulated pulse-height energy distribution spectra for a combination of a lower energy beta emitter and a higher energy beta emitter. The cross-hatched area is that area wherein radioactive decay from beta particles emitted by both the low energy emitter and the high energy emitter contributes to the indicated spectra.
Another phenomenon of interest to a liquid scintillation user is the phenomenon of quenching. All energy created by the beta particle in reaction with the fluor is unfortunately not dissipated into the production of light. The adding of sample material to the scintillation solution introduces molecules which absorb energy and which may not be capable of producing light. The energy of the ejected beta particles, therefore, is dissipated in the excitation of solvent molecules as well as by indirect transfer of energy to obstructive sample molecules. Since some of the energy transferred to sample molecules is not utilized to produce photons of light, it is not measured by the photomultiplier tubes and not recorded. Quenching results in the spectrum of pulse amplitudes having an apparent diminished energy when compared to the theoretical spectrum for an unquenched sample.
In order to relate the actual disintegrations per minute (DPM) of the sample from the measured counts per minute (CPM) it is necessary to determine the extent to which the sample is quenched and correlate the quench level to the counting efficiency. Efficiency of the counting is determined by dividing the actually detected counts per minute (CPM) by the disintegrations per minute (DPM) and is expressed as a fraction. Hence, once the level of quenching is known it is possible to merely divide the actual counts per minute (CPM) by the efficiency fraction to determine the disintegrations per minute (DPM).
Historically, the principal method utilizing a liquid scintillation counter to determine the number of disintegrations per minute for each radionuclide in a solution containing multiple radionuclides is based upon establishing counting conditions in two different energy regions and eliminating the lower energy radionuclide contribution from the higher energy region of the sample. The higher energy radionuclide is counted above the observed maximum energy of the lower energy radionuclide. By discriminating to permit acceptance of any energy event exceeding the maximum energy level of the lower energy beta emitter, an upper level is provided to yield a window or region. This region would include all events exceeding the maximum energy level of the lower energy beta emitting radionuclide and hence would be only events generated by the higher energy beta emitting radionuclide. This region will be referred to as the higher energy region.
The relative quenching of the sample may be determined by using a quench-indicating parameter (QIP) such as the Spectral Index of the External Standard (SIE). This method involves counting the sample with a gamma radiation source adjacent the vial and subsequently in the absence of the gamma radiation source. The gamma radiation source generates Compton electrons in the sample solution which behave in a similar manner to beta particles. If quenching is present the pulse-height energy distribution spectra from the gamma radiation generated events will be compressed towards a lower apparent energy. By measuring the spectral change using a quench-indicating parameter (QIP) such as external standard ratio (ESR), Spectral Index of the External Standard (SIE); H Number, etc., one may obtain the degree of quenching.
The relationship of the degree of quenching of a sample can be equated to the counting efficiency of the higher energy beta emitting radionuclide in the higher energy region using a known series of quench standards. A similar relationship may be established for the higher energy radionuclide in the lower energy region. In like manner the efficiency of the lower energy beta emitting radionuclide may be determined for the lower energy region. Based upon the efficiency for both radionuclides and the combined count rate (CPM), the actual disintegrations per minute for each of the radionuclides may be calculated utilizing simultaneous equations.
The degree of separation of the spectra due to each of the radionuclides varies with quenching. It has been determined that it is possible to maximize nuclide separation by decreasing the energy limits for the region of the higher and lower energy radionuclides as quenching increases. The same type of data is still required for determining counting efficiency. Four plots of counting efficiency related to the quench-indicating parameter of the external standard are required. The data reduction is as set forth above.
The wider the energy region within reason the less likely instrument stability will be a problem. The limit setting for the lower region is in an area of rapidly decreasing spectral intensity for the lower energy radionuclide, hence instrument stability is of a major concern. A change in the instrument could represent a significant change in the apparent counts and efficiency.
In order to provide an improved system, the herein invention is directed towards regionless counting of the combined disintegrations per minute of both radionuclides. The invention is directed towards determining the first moment of the spectrum or mean pulse height or some other quench-indicating parameter (QIP) of a sample mixture and comparing that QIP to the known component QIPs for each of the radionuclides in the mixture at the level of quenching determined for the sample mixture. Based upon this comparison, a relative value may be determined as to the contribution of each of the two radionuclides to the total counts per minute recorded. Utilizing efficiency curves determined for each radionuclide determined from standards and knowing an appropriate sample referenced quench-indicating parameter for the sample mixture relative efficiencies for each of the radionuclides may be determined. Thereafter the disintegrations per minute (DPM) for each radionuclide may be calculated by dividing the counts per minute (CPM) by the efficiencies.
The advantages of utilizing regionless counting include that the user does not need to define an energy region for each radionuclide of interest. In other words the user does not have to determine the maximum energy of the lower energy radionuclide under the appropriate quenched condition to set a region for measuring only the counts per minute in the high energy region contributed by the high energy radionuclide. Potential user error in determining and setting the instrument for these regions is eliminated.
An advantage of utilizing regionless counting is that all the spectral information is used to determine the quench-indicating parameters and to make the desired calculations. Overall theoretical uncertainty using the total spectral information is less than theoretical uncertainty using discrete regions.
Additionally, an advantage of this scheme is that only one set of quench curves per radionuclide needs to be stored in the machine as opposed to one set of quench curves per region. Additionally the counting uncertainties associated with the discrete multiple energy region counting are reduced. The operator does not need to understand the principles involved in liquid scintillation counting to perform automatic dual-label activity measurements but may simply advise the machine to perform such function. Theoretically, more accurate measurements can be made than existing techniques since the entire spectral value information is utilized to reach the appropriate conclusion.
Additionally by using this technique the liquid scintillation counter has the capability of computing both counts per minutes (CPM) and disintegrations per minute (DPM) for individual radionuclides in single and dual-label samples. The liquid scintillation counter may also be set up to provide live time display of the individual count rates and activity levels for the dual-label samples. This scheme additionally contributes to simplified instrument design.